Pressure sensitive coating for image forming

ABSTRACT

A microencapsulated pigment including a leuco dye system is incorporated as one or more layers in a roll of adhesive tape. The tape is applied and may be rubbed by hand or another object to induce a color change. The change may serve as an indicator that the tape is well adhered for masking purposes in a painting operation, or to present a design where only selected portions of the tape are rubbed to induce the color change. The microencapsulated pigment may be constructed so that the color change is irreversible or reversible.

RELATED APPLICATIONS

This application is a continuation-in-part of U.S. patent applicationSer. No. 13/752,056 filed Jan. 28, 2013, and this application is also acontinuation-in-part of International PCT Application Serial No.PCT/US2013/028555 filed Mar. 1, 2013, which designates the United Statesand claims benefit of priority to U.S. Provisional Application Ser. No.61/605,714, filed Mar. 1, 2012, all of which applications are herebyincorporated by reference in their entirety.

BACKGROUND

Carbonless paper was developed in the 1950's (NCR) to satisfy the needof being able to produce a duplicate image of an original document. Theimage is developed when pressure is applied to an original top surface.The pressure generated by the tip of a pen crushes capsules containing aleuco dye releasing the dye which is in solution in one of a number ofpotential solvents. The dye solution then can react with anunencapsulated developer chemical such as an acidified clay or phenoliccompound. The dye becomes protonated and develops a permanent color.Various patents have been granted for microencapsulation processes andcoating processes to manufacture carbonless paper. The extent of the useof carbonless paper has been for producing duplicates of originaldocuments.

Pressure-rupturable microcapsules may be formed in any suitable manner.For example, capsules formed from coacervation of gelatin,polycondensation of urea-formaldehyde, interfacial cross-linking, orhydrolysis of isoclyanatoamidine products may be used. Themicroencapsulation technology is shown generally, by way of example, inU.S. Pat. No. 4,317,743 issued to Chang et al., U.S. Pat. No. 6,620,571issued to Katampe et al., as well as U.S. Pat. No. 6,162,485 issued toChang, all of which are incorporated by reference to the same extent asthough fully replicated herein.

Activities for children may include drawing by various means includingcrayons, water colors, or finger painting. While the children certainlyenjoy these activities, this can necessitate the use of specialprecautions to prevent the children from making an undue mess. Forexample, the activities may be limited to a special area and frequentlyalso close caregiver supervision is required.

Chemicals that change color over a range of temperatures are known asthermochromic systems. Thermochromic chemicals can be manufactured tohave a color change that is reversible or irreversible. U.S. Pat. No.5,591,255, entitled “Thermochromic Ink Formulations, Nail Lacquer andMethods of Use”, issued Jan. 7, 1997 to Small et al., discloses methodsof producing thermochromic coating formulations without ingredientsknown to be harmful to thermochromic inks. The use of distilled water asa fountain solution for off-set printing using thermochromic ink is alsodisclosed.

Thermochromic systems use colorants that are either liquid crystals orleuco dyes. Liquid crystals are used less frequently than leuco dyesbecause they are very difficult to work with and require highlyspecialized printing and handling techniques. Thermochromic pigments area system of interacting parts. Leuco dyes act as colorants, while weakorganic acids act as color developers. Solvents or waxes variablyinteract with the leuco dyes according to the temperature of the system.As is known in the art, thermochromic systems are microencapsulated in aprotective coating to protect the contents from undesired effects fromthe environment. Each microcapsule is self-contained, having all of thecomponents of the entire system that are required for the color change.The components of the system interact with one another differently atdifferent temperatures. Generally, the system is ordered and coloredbelow a temperature corresponding to the full color point. The systembecomes increasingly unordered and starts to lose its color at atemperature corresponding to an activation temperature.

Below the activation temperature, the system is usually colored. Abovethe activation temperature the system is usually clear or lightlycolored. The activation temperature corresponds to a range oftemperatures at which the transition is taking place between the fullcolor point and the clearing point. Generally, the activationtemperature is the temperature at which the human eye can perceive thatthe system is starting to lose color, or alternatively, starting to gaincolor. Presently, thermochromic systems are designed to have activationtemperatures over a broad range, from about −20° C. to about 80° C. ormore. With heating, the system becomes increasingly unordered andcontinues to lose color until it reaches a level of disorder at atemperature corresponding to a clearing point. At the clearing point,the system lacks any recognizable color.

In this manner, thermochromic pigments change from a specific color toclear upon the application of thermal energy or heat in athermally-driven cycle exhibiting well-known hysteresis behavior.Thermochromic pigments come in a variety of colors. When applied to asubstrate, such as paper, the pigment exhibits the color of the dye atthe core of the microcapsules. In one example, when heat is appliedgenerally in the range of 30 to 32° C., the ink changes from the colorof the pigment to clear. When the substrate is allowed to return to atemperature under approximately 30° C., the ink returns to the originalcolor of the pigment.

U.S. Pat. No. 5,785,746, entitled “Preparation Method for Shear-ThinningWater-Based Ball-Point Pen Inks Compositions and Ball-Point PensEmploying the Same,” issued Jul. 28, 1998 to Kito et al., disclosesreversible thermochromic microcapsular pigment mixed in an inkcomposition. The microcapsules have concavities to moderate stressresulting from an external force during use in a ball-point pen.

U.S. Pat. No. 5,805,245, entitled “Multilayered Dispersed ThermochromicLiquid Crystal,” issued Sep. 8, 1998 to Davis, discloses a thermochromicsubstance, applied to inert films in stacked layers with a non-invasivebarrier between each thermochromic substance. The thermochromicsubstance in each layer responds in a different temperature range sothat as the temperature changes, each layer repeats a similar sequenceof colors. The substrate is a water-based acrylic copolymer formulationcoated or permeated with a black pigment. A transparent inert film ornon-invasive barrier serves as a protective coating for thethermochromic film and as a support for the next layer of thethermochromic substance.

Specific thermochromic coating formulations are known in the art. See,for example, U.S. Pat. Nos. 4,720,301, 5,219,625 5,558,700, 5,591,255,5,997,849, 6,139,779, 6,494,950 and 7,494,537, all of which areexpressly incorporated herein by reference. These thermochromic coatingsare known to use various components in their formulations, and aregenerally reversible in their color change. Thermochromic; pigments foruse in these coatings are commercially available in various colors, withvarious activation temperatures, clearing points and full color points.Thermochromic coatings may be printed by offset litho, dry offset,letterpress, gravure, flexo and screen processes, among othertechniques.

SUMMARY

The presently disclosed instrumentalities advance the art by providing aroll of adhesive tape that contains microencapulated pigment intermixedwith one or more layers. This may be used to facilitate paintingoperations where a thermochromic color change confirms that the tape iswell adhered for masking purposes. Alternatively, the tape may beconstructed so that heat or pressure may be used to draw a design onselect areas of the tape.

According to one embodiment, an elongate substrate is formed in a roll.The substrate may be, for example, crepe paper or plastic that presentsa first face and a second face remote from the first face. An adhesivelayer covers the first face of the substrate. A microencapsulatedpigment is intermixed with at least one member of the group consistingof the adhesive layer, a first coating bonded directly to the first faceof the substrate and which is interposed between the substrate and theadhesive layer; and a second coating bonded directly to the second faceof the substrate. The microencapsulated pigment is responsive to atleast one of temperature and pressure to provide a marking that isvisible from a perspective encompassing the second face of thesubstrate.

In one aspect, the roll of tape may be such that the microencapsulatedpigment may be made of frangible capsules as a mixture of differentmicrocapsules respectively incorporating a leuco dye and a developer.The microencapsulated thermochromic pigment responds to pressure thatruptures the microcapsules to provide a pressure-chromic color change byrupturing the capsules. This type of color change is permanent orirreversible.

In one aspect, the roll of tape may be such that the microencapsulatedpigment may be made of capsules that incorporate a leuco dye system withthermochromic functionality. This microencapsulated pigment responds totemperature to provide a thermochromic color change by rupturing thecapsules. This type of color change may be reversible upon cooling ofthe tape, for example, by refrigeration or by the applicant of ice.

Specific applications include adhesive tape that permanently changescolor as it is pressed into position, for example, where the colorchange confirms to a painter that masking tape is actually adhering toan intended position. Another example is interactive decorative usewhere schoolchildren place adhesive tape or film on a desk or textbookand ‘finger paint’ designs.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a substrate that supports one or more layers that contain amixture of microencapsulated components of a leuco dye system wherethese components are released for a color-producing interaction uponrupture of the microcapsule walls.

FIG. 2 shows an image that may be produced by the rupturing of frangiblemicrocapsule walls.

FIG. 3 shows masking tape where the rupture of frangible microcapsulewalls produces a color change which assures the tape is adhering wellfor use in the painting of a wall or floor.

FIGS. 4A, 4B and 4C show a roll of tape wherein the roll of FIG. 4A isunmarked, that of FIG. 4B carries a permanent or non-reversible marking,and that of FIG. 4C carries a temporary or nonreversible marking.

FIG. 5 shows a thermochromic hysteresis curve for a red pigment.

FIG. 6 shows a thermochromic hysteresis curve for a green pigment.

FIG. 7 shows a thermochromic hysteresis curve for a blue pigment.

DETAILED DESCRIPTION

In accordance with the instrumentalities described herein, microcapsulescontaining amine-formaldehyde shell walls are prepared by emulsifying anoily material internal phase in an aqueous medium, and subsequentlyforming amine-formaldehyde walls around the internal phase by in situpolycondensation. A hydrophilic polymer is optionally added to at leastone of the internal phase or the continuous aqueous phase. Thehydrophilic polymer becomes incorporated into the microcapsule wall. Thehydrophilic polymer may be pectin (methylated polygalacturonic acid) ora synthetic hydrophilic polymer, such as a chemically modified gelatin.The hydrophilic polymer may be suitably added to the internal phase inan amount ranging from 0.01 to 10% by weight and more typically about0.15 to 3% based on the monomer and also dependent on the color of theresultant batch.

The hydrophilic polymer is alternatively added to the continuous aqueousphase. The hydrophilic polymer can be dissolved in the continuousaqueous phase where it functions as a viscosity modifier and wallcomponent. Incorporating the hydrophilic polymer into the continuousaqueous phase provides a process for increased control over the size ofthe resultant microcapsules. The increased aqueous phase viscosity leadsto smaller size average capsules. The hydrophilic polymer alsoplasticizes the microcapsule wall thereby providing better stability andcontrol of the dye release mechanism. The amount of hydrophilic polymeradded to the continuous aqueous phase varies with the nature of thehydrophilic polymer and the nature and amount of the other materialsused in the composition. The amount is limited to an amount that doesnot interfere with capsule rupture and reaction of the color former withthe developer. The hydrophilic polymer is preferably incorporated in theaqueous phase in an amount of about 0.01 to 10% by weight based onmonomer used in the composition and more typically in an amount of 0.15to 3%.

Useful hydrophilic polymers include synthetic and natural hydrophilicpolymers. Representative examples of such hydrophilic polymers includegum arabic, gelatin, gelatin derivatives such as phthalated gelatins,cellulose derivatives such as hydroxy cellulose, carboxymethyl celluloseand the like, soluble starches such as dextrin and combinations thereof.A preferred class of hydrophilic polymers is chemically modifiedgelatin. Specific examples of chemically modified gelatins includeGelita™ polymers from Kind & Knox and, more particularly, Gelita™ 8104,8105, 8106 and 8107. These polymers are modified from Type A or Type Bgelatin.

In capsule manufacture, as aqueous phase serves as the continuous phaseof an oil-in-water emulsion in which the oily core materials phase isdispersed. The aqueous phase includes agents known as emulsifiers andsystem modifiers to control the size and uniformity of the microcapsulesand to produce individual mononuclear capsules in preference to clustersof microcapsules. Useful emulsifiers and system modifiers are well knownin the art. Their selection will depend on the type ofmicroencapsulation process used and the nature of the wall formers. Formaking melamine-formaldehyde microcapsules a combination of methylatedpolygalacturonic acid and sulfonated polystyrenes may be used. Thepolygalacturonic acid acts as both a stabilizer and a viscosity modifierfor the aqueous phase, and the sulfonated polystyrenes aid inemulsification.

Typical examples of useful sulfonated polystyrenes are Versa TL500 andVersa TL503, products of National Starch Co. Useful sulfonatedpolystyrenes are generally characterized by a sulfonation degree of over85% and preferably over 95%. The molecular weight of the sulfonatedpolystyrene is preferably greater than 100,000 and more preferably about500,000-1,000,000 but other molecular weights can also be used. Thesulfonated polystyrene is usually added to the aqueous phase in anamount of about 1 to 6% by weight. The quality of this product has alsobeen found to vary with the method by which it is manufactured such thatcertain sulfonated polystyrenes are better than others.

Dye capsules and developer capsules are manufactured separately andsubsequently combined as a mixture. The mixture preferably contains aratio of dye:developer capsules ranging from 1:1 to 1:20 by weight toachieve a pressure sensitive coating of desirable color with minimalresidual color. The following examples teach by way of example and notby limitation.

Example 1 Microencapsulation with Gelatin in the Oil Phase

1. Into a stainless steel beaker, 110 g water and 4.6 g dry sodium saltof polyvinylbenzenesulfonic acid (VERSA) are weighed.

2. The beaker is clamped in place on a hot plate under an overheadmixer. A six-bladed, 45° pitch, turbine impeller is used on the mixer.

3. After thoroughly mixing, 4.0 g pectin (polygalacturonic acid methylester) is slowly sifted into the beaker. This mixture is stirred for 2hours at room temperature (800-1200 rpm).

4. The pH is adjusted to 6.0 with 2% sodium hydroxide.

5. The mixer is turned up to 3000 rpm and the internal phase is addedover a period of 10-15 seconds. Emulsification is continued for 10minutes at a temperature of 25°-30° C.

6. After 20 minutes, the mixing speed is reduced to 2000 rpm, and asolution of melamine-formaldehyde prepolymer is slowly added. Thisprepolymer is prepared by adding 6.5 g formaldehyde solution (37%) to adispersion of 3.9 g melamine in 44 g water. After stirring at roomtemperature for 1 hour the pH is adjusted to 8.5 with 5% sodiumcarbonate and then heated to 62° C. until the solution becomes clear (30minutes).

7. At the start of emulsification, the hot plate is turned up so heatingcontinues during emulsification.

8. The pH is adjusted to 6.0, using 5% phosphoric acid. The beaker isthen covered with foil and placed in a water bath to bring thetemperature of the preparation to 75° C. When 75° C. is reached, the hotplate is adjusted to maintain this temperature for a two hour cure timeduring which the capsule walls are formed.

9. After curing, mixing speed is reduced to 1800 rpm, formaldehydescavenger solution (7.7 g urea and 7.0 g water) is added and thesolution cured another 40 minutes.

10. After 40 minutes hold time, turn down the mixer rpm to 1100 andadjust the pH to 9.5 using a 20% NaOH solution and then allow to stir at500 rpm overnight at room temperature.

The materials forming the internal phase are added in step 5 above, andthe materials forming the aqueous phase are added in step 6. The totalcapsule weight preferably comprises from 5% to 30% of a melamineformaldehyde polymer, or another polymer known to the art that issuitable for microencapsulation. Melamine resin Cas#9003-08-1 isparticularly preferred. The remainder of the capsule constituting 70% to95% of the capsule weight is the internal phase where the internal phaseis formulated either for use as a dye capsule or as a developer capsule.Any system of a leuco dye and developer may be used.

Dye Capsule:

Core Material Wt % Blue Dye Cas# 69898-40-4  1-20% Hexamoll Dinch Cas#166412-78-8 80-99%Developer Capsule:

Core Material Wt % 4,4-Biphenol Cas# 92-88-6  1-25% Isopropyl myristateCas# 110-27-0 75-99%Dye Capsule:

Core Material Wt % Green Dye Cas# 34372-72-0  1-20% Dioctyl phthalateCas# 117-84-0 80-99%Developer Capsule:

Core Material Wt % 4,4-Biphenol Cas# 92-88-6  1-25% Diiso nonylphalate2855-12-0 75-99%

Example 2 Microencapsulation with Gelatin in the Aqueous Phase

Model Laboratory Capsule Preparation

1. Into a stainless steel beaker, 110 g water and 4.6 g dry sodium saltof polyvinylbenzenesulfonic acid (VERSA) are weighed.

2. The beaker is clamped in place on a hot plate under an overheadmixer. A six-bladed, 45° pitch, turbine impeller is used on the mixer.

3. After thoroughly mixing, 4.0 g pectin (polygalacturonic acid methylester) is slowly sifted into the beaker.

4. 0.25-5.0 g gelatin (pellets or solution thereof) is added to thebeaker containing pectin/versa with continuous stirring. This mixture isstirred for 2 hours at room temperature (800-1200 rpm).

5. The pH is adjusted to 6.0 with 2% sodium hydroxide.

6. The mixer is turned up to 3000 rpm and the internal phase is addedover a period of 10-15 seconds. Emulsification is continued for 10minutes at from 25°-30° C.

7. At the start of emulsification, the hot plate is turned up so heatingcontinues during emulsification.

8. After 20 minutes, the mixing speed is reduced to 2000 rpm, and asolution of melamine-formaldehyde prepolymer is slowly added. Thisprepolymer is prepared by adding 6.5 g formaldehyde solution (37%) to adispersion of 3.9 g melamine in 44 g water. After stirring at roomtemperature for 1 hour the pH is adjusted to 8.5 with 5% sodiumcarbonate and then heated to 62° C. until the solution becomes clear (30minutes).

9. The pH is adjusted to 6.0, using 5% phosphoric acid. The beaker isthen covered with foil and placed in a water bath to bring thetemperature of the preparation to 75° C. When 75° C. is reached, the hotplate is adjusted to maintain this temperature for a two hour cure timeduring which the capsule walls are formed.

10. After curing, mixing speed is reduced to 1800 rpm, formaldehydescavenger solution (7.7 g urea and 7.0 g water) is added and thesolution cured another 40 minutes.

11. After 40 minutes hold time, turn down the mixer rpm to 1100 andadjust the pH to 9.5 using a 20% NaOH solution and then allow to stir at500 rpm overnight at room temperature.

The materials forming the internal phase are added in step 6 above, andthe materials forming the aqueous phase are added in step 7. The totalcapsule weight preferably comprises from 5% to 30% of a melamineformaldehyde polymer, or another polymer known to the art that issuitable for microencapsulation. Melamine resin Cas#9003-08-1 isparticularly preferred. The remainder of the capsule constituting 70% to95% of the capsule weight is the internal phase where the internal phaseis formulated either for use as a dye capsule or as a developer capsule.Any system of a leuco dye and developer may be used.

Dye Capsule:

Core Material Wt % Blue Dye Cas# 69898-40-4  1-20% Hexamoll Dinch Cas#166412-78-8 80-99%Developer Capsule:

Core Material Wt % 4,4-Biphenol Cas# 92-88-6  1-25% Isopropyl myristateCas# 110-27-0 75-99%Dye Capsule:

Core Material Wt % Green Dye Cas# 34372-72-0  1-20% Dioctyl phthalateCas# 117-84-0 80-99%Developer Capsule:

Core Material Wt % 4,4-Biphenol Cas# 92-88-6  1-25% Diiso nonylphalate2855-12-0 75-99%

A typical coating composition using the microcapsules described abovecan be coated onto a substrate, such as Mylar or another plastic. Use onpaper or plastic used in the manufacture of adhesive tape isparticularly preferred.

Ingredient Wt (g) Wt %% Microcapsules 4.94 g  29% Phenolic Resin (HRJ4542 from 11.54 g   68% Schenectady Chemical Co.) Polyvinyl alcohol(airvol grade 205 0.26 g 1.5% from Air Products Co.) Sequrez 755(binder) 0.26 g 1.5%

FIG. 1 shows a sheet material 100 that is provided with one or morecoatings made from a mixture of frangible microcapsules as describedabove. A substrate 102 may be, for example, a flexible plastic orcellulosic sheet. A variety of options exist for applied coatings thatcontain a mixture of microcapsules. The microcapsules may be mixed intoa liquid material and applied as layer 104 at the bottom of substrate102. The layer 104 may be applied as a liquid that is then dried orcured to form a solid or gel material. An adhesive layer 106 isoptionally included if it is desirable for the substrate 102 to adhereto other surfaces. If this is the case, then the layer 104 is optionallyeliminated, as a commercially available adhesive may be modified byaddition of the mixture of microcapsules such that color-forming occurswithin the adhesive layer 106. Layer 108 is optionally included oreliminated, and may be a layer like layer 104, except one top-coatingthe substrate 102.

It will be appreciated that any system of commercially available leucodye and developer materials may be used to produce pigments as describedabove in a range of colors. Color options include blue, red, green,black, magenta, orange, aqua, yellow, purple, etc. Color to coloroptions may be green color that develops on yellow, purple color thatdevelops on pink, red color that develops on yellow, etc.

A force 110 may be applied to top surface 112 for purposes of rupturingthe frangible microcapsules. The force 110 may be applied manually usingfingers or manually manipulated tools, such as a spatula or otherimplement. Where this occurs locally, by way of example, it is possibleto drawn an image 200, as shown in FIG. 2, where the substrate 102 is aflexible sheet of plastic or cellulosic material. Where the substrate102 is a masking tape 300 as shown in FIG. 3, the tape may be deployedat the intersection between a wall 302 and a floor 304. Acolor-developed area 306 indicates that the tape has been pressedsufficiently for adherence to the underlying floor 304 or wall 302, andan undeveloped area 308 indicates that the tape has been positioned butadherence is insufficient because the lack of developed color indicatesthe tape has not been pressed against the underlying floor 304 or wall302.

As shown in FIG. 4A, a roll of tape 400 that is prepared as describedabove may be unrolled to present an unmarked face 402. FIG. 4B showsroll 404 presenting a face 406 that has been marked by the use ofpressure sufficient to create a design constituting the words “signhere.” This marking on face 406 is permanent or non-reversible due tothe frangible nature of the microcapsules in layers 104, 106 or 108(shown in FIG. 1).

In an alternative embodiment of FIG. 4C, roll 408 presents a face 410with a design, here shown as a cross-hatch design, that is nonpermanentor reversible due to the use of different microcapsules in layers 104,106, 108.

FIGS. 5-7 show, by way of example, the thermochromic hysteresis behaviorof thermochromic pigments that are formulated for use as describedabove. Taken altogether, FIGS. 5-7 show that thermochromic pigments withdifferent thermal response profiles may be used in combination toprovide variations in color that may differ depending upon the speed atwhich a heated object traverses surface 410 (see FIG. 4C).

FIG. 5 indicates a red pigment response having a color activation alongthe cooling curve from 20° C. to 16.5° C., with full color development500 at 16.5° C. The developed color stays red along the warming cycleuntil color deactivation commences 502 at 29° C. and the color is fullydeactivated 504 at 32° C.

FIG. 6 indicates a green pigment response having a color activationalong the cooling curve from 17° C. to 15° C., with full colordevelopment 600 at 17° C. The developed color stays green along thewarming cycle until color deactivation commences 602 at 26° C. and thecolor is fully deactivated 604 at 28° C.

FIG. 7 indicates a blue pigment response having a color activation alongthe cooling curve from 14° C. to 12° C., with full color development 700at 12° C. The developed color stays blue along the warming cycle untilcolor deactivation commences 702 at about 24° C. and the color is fullydeactivated 704 at 26° C.

Liquid coating materials for use in forming layers 104, 106, 108 may bepurchased on commercial order from Chromatic Technologies, Inc. ofColorado Springs, Colo. The color and thermal hysteresis behavior may beadjusted by design using the principles described below.

Thermochromic coatings useful in the layers 104, 106, 108 containmicrocapsules, which encapsulate a thermochromic system mixed with asolvent. The thermochromic system has a material property of a thermallyconditional hysteresis window that presents a thermal separation. Thesecoatings may be improved according to the instrumentalities describedherein by using a co-solvent that is combined with the thermochromicsystem. The thermochromic system may contain, for example, at least onechromatic organic compound and co-solvents.

One example of a thermochromic system includes a leuco dye having alactone ring structure and a phenolic developer. Within the encapsulatedthermochromic systems, complexes form between the dye and the weak aciddeveloper that allow the lactone ring structure of the leuco dye to beopened. The nature of the complex is such that the hydroxyl groups ofthe phenolic developer interact with the open lactone ring structureforming a supra-molecular structure that orders the dyes and developerssuch that a color is formed. Color forms from this supra-molecularstructure because the dye molecule in the ring open structure iscationic in nature and the molecule has extended conjugation allowingabsorption in the visible spectrum thus producing a colored species. Thecolor that is perceived by the eye is what visible light is not absorbedby the complex. The nature of the dye/developer complex depends on themolar ratio of dye and developer. The stability of the colored complexis determined by the affinity of the solvent for itself, the developeror the dye/developer complex. In a solid state, below the full colorpoint, the dye/developer complex is stable. In the molten state, thesolvent destabilizes the dye/developer complex and the equilibrium ismore favorably shifted towards a developer/solvent complex. This happensat temperatures above the full color point because the dye/developercomplex is disrupted and the extended conjugation of the π cloudelectrons that allow for the absorption of visible light are destroyed.

The melting and crystallization profile of the solvent system determinesthe nature of the thermochromic system. The full color point of thesystem occurs when the maximum amount of dye is developed. In acrystallized solvent state, the dye/developer complex is favored wherethe dye and developer exist in a unique crystallized structure, oftenintercalating with one another to create an extended conjugated πsystem. In the molten state, the solvent(s), in excess, have enoughkinetic energy to disrupt the stability of the dye/developer complex,and the thermochromic system becomes decolorized.

The addition of a co-solvent with a significantly higher melting pointthan the other dramatically changes the melting properties of both thesolvents. By mixing two solvents that have certain properties, a blendcan be achieved that possesses a eutectic melting point. The meltingpoint of a eutectic blend is lower than the melting point of either ofthe co-solvents alone and the melting point is sharper, occurring over asmaller range of temperatures. The degree of the destabilization of thedye/developer complex can be determined by the choice of solvents. Bycreating unique eutectic blends, both the clearing point and the fullcolor point can be altered simultaneously. The degree of hysteresis isthen shifted in both directions simultaneously as the sharpness of themelting point is increased.

Temperature changes in thermochromic systems are associated with colorchanges. If this change is plotted on a graph having axes of temperatureand color, the curves do not align and are offset between the heatingcycle and the cooling cycle. The entire color versus temperature curvehas the form of a loop. Such a result shows that the color of athermochromic system does not depend only on temperature, but also onthe thermal history, i.e. whether the particular color was reachedduring heating or during cooling. This phenomenon is generally referredto as a hysteresis cycle and specifically referred to herein as colorhysteresis or the hysteresis window. Decreasing the width of thishysteresis window to approximately zero would allow for a single valuefor the full color point and a single value for the clearing point. Thiswould allow for a reliable color transition to be observed regardless ofwhether the system is being heated or cooled. Nonetheless, the conceptdecreasing separation across the hysteresis window is elusive inpractice. Thus, it is an object of the present disclosure to providethermochromic systems with a reduced hysteresis window achieved byshifting both the full color point and the clearing point or colordeactivation temperature, for example.

Leuco Dyes

Leuco dyes most commonly used as color formers in thermochromic systemsof the present disclosure include, but are not limited to, generally;spirolactones, fluorans, spiropyrans, and fulgides; and morespecifically; diphenylmethane phthalide derivatives,phenylindolylphthalide derivatives, indolylphthalide derivatives,diphenylmethane azaphthalide derivatives, phenylindolylazaphthalidederivatives, fluoran derivatives, styrynoquinoline derivatives, anddiaza-rhodamine lactone derivatives which can include:3,3-bis(p-dimethylaminophenyl)-6-dimethylaminophthalide;3-(4-diethylaminophenyl)-3-(1-ethyl-2-methylindol-3-yl) phthalide;3,3-bis(1-n-butyl-2-methylindol-3-yl)phthalide;3,3-bis(2-ethoxy-4-diethylaminophenyl)-4-azaphthalide;3-[2-ethoxy-4-(N-ethylanilino)phenyl]-3-(1-ethyl-2-methylindol-3-yl)-4-azaphthalide;3,6-dimethoxyfluoran; 3,6-di-n-butoxyfluoran;2-methyl-6-(N-ethyl-N-p-tolylamino)fluoran;3-chloro-6-cyclohexylaminofluoran; 2-methyl-6-cyclohexylaminofluoran;2-(2-chloroanilino)-6-di-n-butylamino fluoran;2-(3-trifluoromethylanilino)-6-diethylaminofluoran;2-(N-methylanilino)-6-(N-ethyl-N-p-tolylamino) fluoran,1,3-dimethyl-6-diethylaminofluoran; 2-chloro-3-methyl-6-diethylaminofluoran; 2-anilino-3-methyl-6-diethylaminofluoran;2-anilino-3-methyl-6-di-n-butylamino fluoran;2-xylidino-3-methyl-6-diethylaminofluoran;1,2-benzo-6-diethylaminofluoran;1,2-benzo-6-(N-ethyl-N-isobutylamino)fluoran,1,2-benzo-6-(N-ethyl-N-isoamylamino)fluoran;2-(3-methoxy-4-dodecoxystyryl)quinoline; spiro[5H-(1)benzopyrano(2,3-d)pyrimidine-5,1′(3′H)isobenzofuran]-3′-one;2-(diethylamino)-8-(diethylamino)-4-methyl-spiro[5H-(1)benzopyrano(2,3-d)pyrimidine-5,1′(3′H)isobenzofuran]-3′-one;2-(di-n-butylamino)-8-(di-n-butylamino)-4-methyl-spiro[5H-(1)benzopyrano(2,3-d)pyrimidine-5,1′(3′H)isobenzofuran]-3′-one;2-(di-n-butylamino)-8-(diethylamino)-4-methyl-spiro[5H-(1)benzopyrano(2,3-d)pyrimidine-5,1′(3′H)isobenzofuran]-3′-one;2-(di-n-butylamino)-8(N-ethyl-N-isoamylamino)-4-methyl-spiro[5H-(1)benzopyrano(2,3-d)pyrimidine-5,1′(3′H)isobenzofuran]-3′-one;and 2-(di-n-butylamino)-8-(di-n-butylamino)-4-phenyl and trisubstitutedpyridines.

Developers

Weak acids that can be used as color developers act as proton donors,changing the dye molecule between its leuco form and its protonatedcolored form; stronger acids make the change irreversible. Examples ofdevelopers used in the present disclosure include but are not limitedto: bisphenol A; bisphenol F; tetrabromobisphenol A;1′-methylenedi-2-naphthol; 1,1,1-tris(4-hydroxyphenyl)ethane;1,1-bis(3-cyclohexyl-4-hydroxyphenyl)cyclohexane;1,1-bis(4-hydroxy-3-methylphenyl)cyclohexane;1,1-bis(4-hydroxyphenyl)cyclohexane;1,3-bis[2-(4-hydroxyphenyl)-2-propyl]benzene; 1-naphthol; 2-naphthol;2,2 bis(2-hydroxy-5-biphenylyl)propane;2,2-bis(3-cyclohexyl-4-hydroxy)propane;2,2-bis(3-sec-butyl-4-hydroxyphenyl)propane;2,2-bis(4-hydroxy-3-isopropylphenyl)propane;2,2-bis(4-hydroxy-3-methylphenyl)propane;2,2-bis(4-hydroxyphenyl)propane; 2,3,4-trihydroxydiphenylmethane;4,4′-(1,3-Dimethylbutylidene)diphenol; 4,4′-(2-Ethylidene)diphenol;4,4′-(2-hydroxybenzylidene)bis(2,3,6-trimethylphenol); 4,4′-biphenol;4,4′-dihydroxydiphenyl ether; 4,4′-dihydroxydiphenylmethane;4,4′-methylidenebis(2-methylphenol); 4-(1,1,3,3-tetramethylbutyl)phenol;4-phenylphenol; 4-tert-butylphenol; 9,9-bis(4-hydroxyphenyl)fluorine;4,4′-(ethane-1,1-diyfldiphenol;alpha,alpha′-bis(4-hydroxyphenyl)-1,4-diisopropylbenzene;alpha,alpha,alpha′-tris(4-hydroxyphenyl)-1-ethyl-4-isopropylbenzene;benzyl 4-hydroxybenzoate; bis(4-hydroxyphenyl)sulfide;bis(4-hydroxyphenyl)sulfone; propyl 4-hydroxybenzoate; methyl4-hydroxybenzoate; resorcinol; 4-tert-butyl-catechol;4-tert-butyl-benzoic acid; 1,1′-methylenedi-2-naphthol1,1,1-tris(4-hydroxyphenyl)ethane;1,1-bis(3-cyclohexyl-4-hydroxyphenyl)cyclohexane;1,1-bis(4-hydroxy-3-methylphenyl)cyclohexane;1,1-bis(4-hydroxyphenyl)cyclohexane;1,3-bis[2-(4-hydroxyphenyl)-2-propyl]benzene; 1-naphthol 2,2′-biphenol;2,2-bis(2-hydroxy-5-biphenylyl)propane;2,2-bis(3-cyclohexyl-4-hydroxyphenyl)propane;2,2-bis(3-sec-butyl-4-hydroxyphenyl)propane;2,2-bis(4-hydroxy-3-isopropylphenyl)propane;2,2-bis(4-hydroxy-3-methylphenyl)propane;2,2-bis(4-hydroxyphenyl)propane; 2,3,4-trihydroxydiphenylmethane;2-naphthol; 4,4′-(1,3-dimethylbutylidene)diphenol;4,4′-(2-ethylhexylidene)diphenol;4,4′-(2-hydroxybenzylidene)bis(2,3,6-trimethylphenol); 4,4′-biphenol;4,4′-dihydroxydiphenyl ether; 4,4′-dihydroxydiphenylmethane;4,4′-ethylidenebisphenol; 4,4′-methylenebis(2-methylphenol);4-(1,1,3,3-tetramethylbutyl)phenol; 4-phenylphenol; 4-tert-butylphenol;9,9-bis(4-hydroxyphenyl)fluorine;alpha,alpha′-bis(4-hydroxyphenyl)-1,4-diisopropylbenzene;α,α,α-tris(4-hydroxyphenyl)-1-ethyl-4-isopropylbenzene; benzyl4-hydroxybenzoate; bis(4-hydroxyphenyl) sulfidem; bis(4-hydroxyphenyl)sulfone methyl 4-hydroxybenzoate; resorcinol; tetrabromobisphenol A;3,5-di-tertbutyl-salicylic acid; zinc 3,5-di-tertbutylsalicylate;3-phenyl-salicylic acid; 5-tertbutyl-salicylic acid; 5-n-octyl-salicylicacid; 2,2′-biphenol; 4,4′-di-tertbutyl-2,2′-biphenol;4,4′-di-n-alkyl-2,2′-biphenol; and 4,4′-di-halo-2,2′-biphenol, whereinthe halo is chloro, fluoro, bromo, or iodo.

Solvents

The best solvents to use within the thermochromic system are those thathave low reactivity, have a relatively large molecular weight (i.e. over100), and which are relatively non-polar. Very low molecular weightaldehydes, ketones, diols and aromatic compounds should not be used assolvents within the thermochromic system.

Thermochromic coatings disclosed herein use a co-solvent that iscombined with the thermochromic system. This material may be provided inan effective amount to reduce the thermal separation in the overallcoating to a level less than eighty percent of separation that wouldotherwise occur if the material were not added. This effective amountmay range, for example from the 12% to 15% by weight of the composition.

The addition of a co-solvent with a significantly higher melting pointthan the other dramatically changes the melting properties of both thesolvents. By mixing two solvents that have certain properties, a blendcan be achieved that possesses a eutectic melting point. The meltingpoint of a eutectic blend is lower than the melting point of either ofthe co-solvents alone and the melting point is sharper, occurring over asmaller range of temperatures. The degree of the destabilization of thedye/developer complex can be determined by the choice of solvents. Bycreating unique eutectic blends, both the clearing point and the fullcolor point can be altered simultaneously. The degree of hysteresis isthen shifted in both directions simultaneously as the sharpness of themelting point is increased. Copending application Ser. No. 13/363,070filed Jan. 31, 2012 discloses thermochromic systems with controlledhysteresis, and is hereby incorporated by reference to the same extentas though fully replicated herein. According to the instrumentalitiesdescribed therein, the microencapsulate pigments may be formulated tohave color transition temperatures across a hysteresis window of lessthan five degrees centigrade or more than 60 or 80 degrees centigrade.

Properties of at least one of the co-solvents used in the presentdisclosure include having a long fatty tail of between 12 and 24 carbonsand possessing a melting point that is about 70° C. to about 200° C.greater than the co-solvent partner. The co-solvents are preferably alsocompletely miscible at any ratio.

Solvents and/or co-solvents used in thermochromic generally may include,but are not limited to, sulfides, ethers, ketones, esters, alcohols, andacid amides. These solvents can be used alone or in mixtures of 2 ormore. Examples of the sulfides include: di-n-octyl sulfide; di-n-nonylsulfide; di-n-decyl sulfide; di-n-dodecyl sulfide; di-n-tetradecylsulfide; di-n-hexadecyl sulfide; di-n-octadecyl sulfide; octyl dodecylsulfide; diphenyl sulfide; dibenzyl sulfide; ditolyl sulfide;diethylphenyl sulfide; dinaphthyl sulfide; 4,4′-dichlorodiphenylsulfide; and 2,4,5,4′tetrachlorodiphenyl sulfide. Examples of the ethersinclude: aliphatic ethers having 10 or more carbon atoms, such asdipentyl ether, dihexyl ether, diheptyl ether, dioctyl ether, dinonylether, didecyl ether, diundecyl ether, didodecyl ether, ditridecylether, ditetradecyl ether, dipentadecyl ether, dihexadecyl ether,dioctadecyl ether, decanediol dimethyl ether, undecanediol dimethylether, dodecanediol dimethyl ether, tridecanediol dimethyl ether,decanediol diethyl ether, and undecanediol diethyl ether; alicyclicethers such as s-trioxane; and aromatic ethers such as phenylether,benzyl phenyl ether, dibenzyl ether, di-p-tolyl ether,1-methoxynaphthalene, and 3,4,5trimethoxytoluene.

Examples of ketone solvents include: aliphatic ketones having 10 or morecarbon atoms, such as 2-decanone, 3-decanone, 4-decanone, 2-undecanone,3-undecanone, 4-undecanone, 5-undecanone, 6-undecanone, 2-dodecanone,3-dodecanone, 4-dodecanone, 5-dodecanone, 2-tridecanone, 3-tridecanone,2-tetradecanone, 2-pentadecanone, 8-pentadecanone, 2-hexadecanone,3-hexadecanone, 9-heptadecanone, 2-pentadecanone, 2-octadecanone,2-nonadecanone, 10-nonadecanone, 2-eicosanone, 11-eicosanone,2-heneicosanone, 2-docosanone, laurone, and stearone; aryl alkyl ketoneshaving 12 to 24 carbon atoms, such as n-octadecanophenone,n-heptadecanophenone, n-hexadecanophenone, n-pentadecanophenone,n-tetradecanophenone, 4-n-dodecaacetophenone, n-tridecanophenone,4-n-undecanoacetophenone, n-laurophenone, 4-n-decanoacetophenone,n-undecanophenone, 4-n-nonylacetophenone, n-decanophenone,4-n-octylacetophenone, n-nonanophenone, 4-n-heptylacetophenone,n-octanophenone, 4-n-hexylacetophenone, 4-n-cyclohexylacetophenone,4-tert-butylpropiophenone, n-heptaphenone, 4-n-pentylacetophenone,cyclohexyl phenyl ketone, benzyl n-butyl ketone, 4-n-butylacetophenone,n-hexanophenone, 4-isobutylacetophenone, 1-acetonaphthone,2-acetonaphthone, and cyclopentyl phenyl ketone; aryl aryl ketones suchas benzophenone, benzyl phenyl ketone, and dibenzyl ketone; andalicyclic ketones such as cyclooctanone, cyclododecanone,cyclopentadecanone, and 4-tert-butylcyclohexanone, ethyl caprylate,octyl caprylate, stearyl caprylate, myristyl caprate, stearyl caprate,docosyl caprate, 2-ethylhexyl laurate, n-decyl laurate, 3-methylbutylmyristate, cetyl myristate, isopropyl palmitate, neopentyl palmitate,nonyl palmitate, cyclohexyl palmitate, n-butyl stearate, 2-methylbutylstearate, stearyl behenate 3,5,5-trimethylhexyl stearate, n-undecylstearate, pentadecyl stearate, stearyl stearate, cyclohexylmethylstearate, isopropyl behenate, hexyl behenate, lauryl behenate, behenylbehenate, cetyl benzoate, stearyl p-tert-butylbenzoate, dimyristylphthalate, distearyl phthalate, dimyristyl oxalate, dicetyl oxalate,dicetyl malonate, dilauryl succinate, dilauryl glutarate, diundecyladipate, dilauryl azelate, di-n-nonyl sebacate,1,18-dineopentyloctadecylmethylenedicarboxylate, ethylene glycoldimyristate, propylene glycol dilaurate, propylene glycol distearate,hexylene glycol dipalmitate, 1,5-pentanediol dimyristate,1,2,6-hexanetriol trimyristate, 1,4-cyclohexanediol didecanoate,1,4-cyclohexanedimethanol dimyristate, xylene glycol dicaprate, andxylene glycol distearate.

Ester solvents can be selected from esters of a saturated fatty acidwith a branched aliphatic alcohol, esters of an unsaturated fatty acidor a saturated fatty acid having one or more branches or substituentswith an aliphatic alcohol having one or more branches or 16 or morecarbon atoms, cetyl butyrate, stearyl butyrate, and behenyl butyrateincluding 2-ethylhexyl butyrate, 2-ethylhexyl behenate, 2-ethylhexylmyristate, 2-ethylhexyl caprate, 3,5,5-trimethylhexyl laurate,3,5,5-trimethylhexyl palmitate, 3,5,5-trimethylhexyl stearate,2-methylbutyl caproate, 2-methylbutyl caprylate, 2-methylbutyl caprate,1-ethylpropyl palmitate, 1-ethylpropyl stearate, 1-ethylpropyl behenate,1-ethylhexyl laurate, 1-ethylhexyl myristate, 1-ethylhexyl palmitate,2-methylpentyl caproate, 2-methylpentyl caprylate, 2-methylpentylcaprate, 2-methylpentyl laurate, 2-methylbutyl stearate, 2-methylbutylstearate, 3-methylbutyl stearate, 2-methylheptyl stearate, 2-methylbutylbehenate, 3-methylbutyl behenate, 1-methylheptyl stearate,1-methylheptyl behenate, 1-ethylpentyl caproate, 1-ethylpentylpalmitate, 1-methylpropyl stearate, 1-methyloctyl stearate,1-methylhexyl stearate, 1,1dimethylpropyl laurate, 1-methylpentylcaprate, 2-methylhexyl palmitate, 2-methylhexyl stearate, 2-methylhexylbehenate, 3,7-dimethyloctyl laurate, 3,7-dimethyloctyl myristate,3,7-dimethyloctyl palmitate, 3,7-dimethyloctyl stearate,3,7-dimethyloctyl behenate, stearyl oleate, behenyl oleate, stearyllinoleate, behenyl linoleate, 3,7-dimethyloctyl erucate, stearylerucate, isostearyl erucate, cetyl isostearate, stearyl isostearate,2-methylpentyl 12-hydroxystearate, 2-ethylhexyl 18-bromostearate,isostearyl 2-ketomyristate, 2-ethylhexyl-2-fluoromyristate, cetylbutyrate, stearyl butyrate, and behenyl butyrate.

Examples of the alcohol solvents include monohydric aliphatic saturatedalcohols such as decyl alcohol, undecyl alcohol, dodecyl alcohol,tridecyl alcohol, tetradecyl alcohol, pentadecyl alcohol, hexadecylalcohol, heptadecyl alcohol, octadecyl alcohol, eicosyl alcohol, behenylalcohol and docosyl alcohol; aliphatic unsaturated alcohols such asallyl alcohol and oleyl alcohol, alicyclic alcohols such ascyclopentanol, cyclohexanol, cyclooctanol, cyclododecanol, and4-tert-butylcyclohexanol; aromatic alcohols such as 4-methylbenzylalcohol and benzhydrol; and polyhydric alcohols such as polyethyleneglycol. Examples of the acid amides include acetamide, propionamide,butyramide, capronamide, caprylamide, capric amide, lauramide,myristamide, palmitamide, stearamide, behenamide, oleamide, erucamide,benzamide, capronanilide, caprylanilide, capric anilide, lauranilide,myristanilide, palmitanilide, stearanilide, behenanilide, oleanilide,erucanilide, N-methylcapronamide, N-methylcaprylamide, N-methyl (capricamide), N-methyllauramide, N-methylmyristamide, N-methylpalmitamide,N-methylstearamide, N-methylbehenamide, N-methyloleamide,N-methylerucamide, N-ethyllauramide, N-ethylmyristamide,N-ethylpalmitamide, N-ethylstearamide, N-ethyloleamide,N-butyllauramide, N-butylmyristamide, N-butylpalmitamide,N-butylstearamide, N-butyloleamide, N-octyllauramide,N-octylmyristamide, N-octylpalmitamide, N-octylstearamide,N-octyloleamide, N-dodecyllauramide, N-dodecylmyristamide,N-dodecylpalmitamide, N-dodecylstearamide, N-dodecyloleamide,dilauroylamine, dimyristoylamine, dipalmitoylamine, distearoylamine,dioleoylamine, trilauroylamine, trimyristoylamine, tripalmitoylamine,tristearoylamine, trioleoylamine, succinamide, adipamide, glutaramide,malonamide, azelamide, maleamide, N-methylsuccinamide, N-methyladipamide, N-methylglutaramide, N-methylmalonamide, N-methylazelamide,N-ethylsuccinamide, N-ethyladipamide, N-ethylglutaramide,N-ethylmalonamide, N-ethylazelamide, N-butylsuccinamide,N-butyladipamide, N-butylglutaramide, N-butylmalonamide,N-octyladipamide, and N-dodecyladipamide.

Among these solvents, it has been discovered that certain solvents havethe effect of reducing the hysteresis window. The solvent may bematerial combined with the thermochromic system, for example, to reducethermal separation across the hysteresis window to a level demonstrating80%, 70%, 50%, 40%, 30% or less of the thermal separation that wouldexist if the co-solvent were not present. The co-solvent is selectedfrom the group consisting of derivatives of mysristic acid, derivativesof behenyl acid, derivatives of palmytic acid and combinations thereof.Generally, these materials include myristates, palmitates, behenates,together with myristyl, stearyl, and behenyl materials and certainalcohols. In one aspect, these materials are preferably solvents andco-solvents from the group including isopropyl myristate, isopropylpalmitate, methyl palmitate, methyl stearate, myristyl myristate, cetylalcohol, stearyl alcohol, behenyl alcohol, stearyl behenate, andstearamide. These co-solvents are added to the encapsulatedthermochromic system in an amount that, for example, ranges from 9% to18% by weight of the thermochromic system as encapsulated, i.e.,excluding the weight of the capsule. This range is more preferably fromabout 12% to about 15% by weight.

Light Stabilizers

Thermochromic coatings containing leuco dyes are available for all majorcoating types such as water-based, ultraviolet cured and epoxy. Theproperties of these coatings differ from process coatings. For example,most thermochromic coatings contain the thermochromic systems asmicrocapsules, which are not inert and insoluble as are ordinary processpigments. The size of the microcapsules containing the thermochromicsystems ranges typically between 3-5 μm which is more than 10-timeslarger than regular pigment particles found in most coatings. Thepost-print functionality of thermochromic coatings can be adverselyaffected by ultraviolet light, temperatures in excess of 140° C. andaggressive solvents. The lifetime of these coatings is sometimes verylimited because of the degradation caused by exposure to ultravioletlight from sunlight.

In other instances, additives used to fortify the encapsulatedthermochromic systems by imparting a resistance to degradation byultraviolet light by have a dual functionality of also reducing thewidth of separation over the hysteresis window. Light stabilizers areadditives which prevent degradation of a product due to exposure toultraviolet radiation. Examples of light stabilizers used inthermochromic systems of the present disclosure and which may alsoinfluence the hysteresis window include but are not limited to:avobenzone, bisdisulizole disodium, diethylaminohydroxybenzoyl hexylbenzoate, Ecamsule, methyl anthranilate, 4-aminobenzoic acid, Cinoxate,ethylhexyl triazone, homosalate, 4-methylbenzylidene camphor, octylmethoxycinnamate, octyl salicylate, Padimate O, phenylbenzimidazolesulfonic acid, polysilicone-15, trolamine salicylate, bemotrizinol,benzophenones 1-12, dioxybenzone, drometrizole trisiloxane,iscotrizinol, octocrylene, oxybenzone, sulisobenzone, bisoctrizole,titanium dioxide and zinc oxide.

Careful preparation of encapsulated reversible thermochromic materialenhances coating stability in the presence of low molecular weight polarsolvents that are known to adversely affect thermochromic behavior. Oneskilled in the art of microencapsulation can utilize well-knownprocesses to enhance the stability of the microcapsule. For example, itis understood that increasing the cross linking density will reduce thepermeability of the capsule wall, and so also reduces the deleteriouseffects of low molecular weight solvents. It is also commonly understoodthat, under certain conditions, weak acids with a pKa greater than about2 may catalyze microcapsule wall polymerization and increase theresulting cross linking density. It is presently the case that usingformic acid as a catalyst enhances solvent stability of bluethermochromic microcapsules in the presence of low molecular weightketones, diols, and aldehydes at room temperature. Further, it is wellunderstood that increasing the diameter of the thermochromicmicrocapsule can result in enhanced solvent stability.

The selection of material for use as the non-polar solvent for thethermochromic dye and color developer that is encapsulated within thethermochromic pigment determines the temperature at which color changeis observed. For example, changing the solvent from a single componentto a two component solvent system can shift the temperature at whichfull color is perceived almost 7° C. from just under 19° C. to 12° C.The present disclosure shows how to apply this knowledge in preparingresin-based vehicle coatings for use in can and coil coatings with fullcolor temperatures, i.e., the temperature at which maximum colorintensity is observed, as low as −5° C. and as high as 65° C. No adverseeffects on the physical properties of the resulting coating wereobserved as the full color temperature was changed over the above rangeby the use of different straight chain alkyl esters, alcohols, ketonesor amides.

Thermochromic materials including encapsulated thermochromic systemswith a variety of color properties may be purchased on commercial orderfrom such companies as Chromatic Technologies, Inc., of ColoradoSprings, Colo.

Control over observed color intensity is demonstrated in several ways,generally by providing increased amounts of pigment. For a typicalcoating, material thickness ranges from 1 mg/in2 to 6 mg/in2. Veryintense color is observed for coatings with thickness greater than about3 mg/in2. Increasing thermochromic pigment solids can also result in amore intense observed color even when coating thickness is decreased.However, dried film properties such as flexibility and toughness may becompromised if too much thermochromic pigment is incorporated. Theoptimal range of thermochromic pigment solids is within 5 to 40% byweight of the coating.

Encapsulation Process for Non-Frangible Capsules

Nearly all thermochromic systems require encapsulation for protection.As is known in the art, the most common process for encapsulation isinterfacial polymerization. During interfacial polymerization theinternal phase (material inside the capsule), external phase (wallmaterial of the capsule) and water are combined through high-speedmixing. By controlling all the temperature, pH, concentrations, andmixing speed precisely, the external phase will surround the internalphase droplet while crosslinking with itself. Usually the capsules arebetween 3-5 μm or smaller. Such small sizes of capsules are referred toas microcapsules and the thermochromic system within the microcapsulesare microencapsulated. Microencapsulation allows thermochromic systemsto be used in wide range of materials and products. The size of themicrocapsules requires some adjustments to suit particular printing andmanufacturing processes.

The size distribution of microcapsules can range from as much as 0.2 μmto 100 μm. Further example techniques of physical microencapsulationinclude but are not limited to pan coating, air suspension coating,centrifugal extrusion, vibration nozzle, and spray drying. Examples ofchemical microencapsulation techniques include but are not limited tointerfacial polymerization, in-situ polymerization, and matrixpolymerization. Example polymers used in the preferred chemicalmicroencapsulation include but are not limited to polyester,polyurethane, polyureas, urea-formaldehyde, epoxy,melamine-formaldehyde, polyethylene, polyisocyanates, polystyrene,polyamides, and polysilanes.

The capsule isolates the thermochromic system from the environment, butthe barrier that the capsule provides is itself soluble to certainsolvents. Therefore, the microcapsule constituents interact with theenvironment to some extent. The solubility parameter describes how mucha material will swell in the presence of different solvents. Thisswelling will directly impact the characteristics of the reactionpotential within the capsule, as well as potentially making the capsulemore permeable, both of which will likely adversely affect thethermochromic system. Solvents in which the microcapsules are exposed toare chosen so as not to destroy, or affect, the thermochromic systemwithin.

The capsule is hard, thermally stable and relatively impermeable. Theinfiltration of compounds through the capsule are stopped or slowed tothe point that the characteristics of the dye are not affected. Thepollution of the thermochromic system within the capsule by solventsfrom the environment affects the shelf life of the thermochromic system.Therefore, the formulation of the applied thermochromic system, as acoating for example, should be carefully considered.

In an embodiment of the present disclosure, capsules are made from ureaformaldehyde. One technique used to produce the encapsulatedthermochromic systems is to combine water, dye, oil, and ureaformaldehyde and mix to create a very fine emulsification. Because ofthe properties of the compounds, the oil and dye end up on the inside ofthe capsule and the water ends up on the outside, with the ureaformaldehyde making up the capsule itself. The capsule can then bethermo-set, similar to other resins, such as formica. The thermo-setsubstance is very hard and will not break down, even at temperatureshigher than the encapsulated thermochromic system is designed to beexposed to. The urea formaldehyde capsule is almost entirely insolublein most solvents, but it is permeable to certain solvents that mightdestroy the ability of the thermochromic system to color and decolorizethroughout a temperature range.

The extent to which capsules will react with their environment isinfluenced by the pH of the surrounding medium, the permeability of thecapsule, the polarity and reactivity of compounds in the medium, and thesolubility of the capsule. Preferred media used in formulatingencapsulated thermochromic system are engineered to reduce thereactivity between that medium and the capsules to a low enough levelthat the reactivity will not influence the characteristics of the dyefor an extended period of time.

Highly polar solvent molecules, with the exception of water, ofteninteract more with the leuco dye than with the capsule shell and othernon-polar molecules of the thermochromic system. Therefore, polarsolvents that are able to cross the capsule barrier should, in general,be eliminated from the medium within which the encapsulatedthermochromic system is formulated.

Aqueous media that the encapsulated thermochromic systems are placedwithin should have a narrow pH range from about 6.5 to about 7.5. Whenan encapsulated thermochromic system is added to a formulation that hasa pH outside this range, often the thermochromic properties of thesystem are destroyed. This is an irreversible effect.

One aspect of the present disclosure is for a method of improving theformulations of the thermochromic system by removing any aldehydes,ketones, and diols and replacing them with solvents which do notadversely affect the thermochromic system. Solvents having a largemolecular weight (i.e. greater than 100) generally are compatible withthe thermochromic systems. The acid content of the system is preferablyadjusted to an acid number below 20 or preferably adjusted to beneutral, about 6.5-7.5. Implementing these solvent parameters for use inthe thermochromic system will preserve the reversible coloration abilityof the leuco dyes.

Formulations for thermochromic systems are engineered with all theconsiderations previously mentioned. The examples below describe athermochromic system with excellent color density, low residual color,narrow temperature ranges between full color and clearing point, and anarrow hysteresis window. The full color point and the clearing pointare determined by visual inspection of the thermochromic system at arange of temperatures. The difference in temperature between the maximaof color change during the cooling cycle and the heating cycle is usedto calculate hysteresis.

Vehicle

Physical properties of the finished coating can be significantlyaffected by the selection of resin to be used. When no resin is used informulating a reversible thermochromic coating, a matte finish isachieved that is able to be formed into can ends, tabs, caps and/orother closures. While this result may be desired, the inclusion of a lowviscosity, relatively low molecular weight resin, monomer, oligomer,polymer, or combination thereof, can enhance gloss and affect otherphysical film properties such as hardness, flexibility and chemicalresistance. The resin is designed to supplement the total solidsdeposited on the substrate, thus impacting the physical properties ofthe dried film. Any resin material, monomer, oligomer, polymer, orcombination thereof that can be polymerized into the commerciallyavailable can and coil coating material is suitable for inclusion in theformulation of the current reversible thermochromic can and coilcoating. Acceptable classes of resins include, but are not limited topolyester, urethane, acrylic acid and acrylate, or other types of resinsystems with suitably high solids content.

Adjusting the Acid Content

Water-based coatings are pH adjusted prior to addition of thermochromicpigment. As mentioned above, the pH should be neutral unless observationindicates that a different pH is required. To achieve the correct pH,one uses a good proton donor or acceptor, depending on whether the pH isto be adjusted up or down. To lower the pH, sulfuric acid is used, toraise it, the best proton acceptor so far is KOH. These two chemicalsare very effective and do not seem to impart undesirable characteristicsto the medium. The most effective pH is about 7.0, however, sometolerance has been noted between 6.0 and 8.0. A pH below 6.0 and above8.0 has almost always immediately destroyed the pigment.

The acid value is defined as the number of milligrams of a 0.1 N KOHsolution required to neutralize the alkali reactive groups in 1 gram ofmaterial under the conditions of ASTM Test Method D-1639-70. It is notyet fully understood how non-aqueous substances containing acid affectthe thermochromic, but high acid number substances have inactivated thethermochromic pigments. Generally, the lower the acid number the better.To date coating formulations with an acid value below 20 and notincluding the harmful solvents described above have worked well. Somehigher acid value formulations may be possible but generally it is bestto use vehicle ingredients with low acid numbers or to adjust the acidvalue by adding an alkali substance. The greatest benefit of a neutralor low acid value vehicle will be increased shelf life. Buffers havebeen used historically in offset coating formulations to minimize theeffects of the fountain solution on pigment particles. This is onepossible solution to the potential acidity problem of the varnishes. Oneingredient often used as a buffer is cream of tartar. A dispersion ofcream of tartar and linseed oil can be incorporated into the coating.The net effect is that the pigments in the coating are protected fromthe acidic fountain solution.

Coating Formulations

The encapsulated thermochromic systems of the present disclosure may bereferred to as pigments. In order to add normal pigment to coating, dye,or lacquer, the pigment itself is ground into the base. This dispersesthe pigment throughout the base. The addition of more pigmentintensifies the color. Since the pigment often has a very intense color,it is sometimes acceptable for only about 10% of the final coating to bemade up of normal pigments.

A base for a coating formulation using encapsulated thermochromicsystems of the present disclosure may be developed using off the shelfingredients. The coating will incorporate, where possible, and becompatible with different coating types and solvents with molecularweights larger than 100 while avoiding aldehydes, diols, ketones, and,in general, aromatic compounds Important considerations with respect tothe ingredients within the coating vehicle are the reactivity of theingredients with the encapsulated thermochromic system.

Unwanted interactions between media and the encapsulated thermochromicsystems can occur between compounds found in coating formulations. Thelong alkyl chains of many of the compounds found in coating vehicles mayhave reactive portions that can fit through the pores of the capsule andinteract with the inner phase and denature it through this interaction.Since the behavior of the thermochromic system is related to the shapeand the location of its molecules at given temperatures, disruptingthese structures could have a large impact on the characteristics of thethermochromic system. Even molecules that cannot fit through the capsulepores may have reactive portions that could protrude into the capsuleand thereby influence the color transition of the thermochromic systemwithin the capsule. Therefore, mineral spirits, ketones, diols, andaldehydes are preferably minimized in any medium in which theencapsulated are also preferably avoided. If these compounds aresubstantially reduced or eliminated the thermochromic systems willperform better and have a longer shelf life.

Another important step in using the encapsulated thermochromic systemsof the present disclosure in coating formulations is to adjust the pH orlower the acid value of the coating base before the thermochromic systemis added. This can be done by ensuring that each individual component ofthe base is at the correct pH or acid value or by simply adding a protondonor or proton acceptor to the base itself prior to adding thethermochromic system. The appropriate specific pH is generally neutral,or 7.0. The pH will vary between 6.0 and 8.0 depending on the coatingtype and the color and batch of the thermochromic system.

Once a slurry and the base have been properly prepared, they arecombined. The method of stirring should be low speed with non-metal stirblades. Other additives may be incorporated to keep the thermochromicsystem suspended. The coating should be stored at room temperature.

Most thermochromic pigments undergo a color change from a specific colorto colorless. Therefore, layers of background colors can be providedunder thermochromic layers that will only be seen when the thermochromicpigment changes to colorless. If an undercoat of yellow is applied tothe substrate and then a layer containing blue thermochromic pigment isapplied the color will appear to change from green to yellow, when whatis really happening is that the blue is changing to colorless.

The substrates that the thermochromic coatings are printed upon arepreferably neutral in pH, and should not impart any chemicals to thecapsule that will have a deleterious effect on it.

Thermochromic coatings contain, in combination, a vehicle and a pigmentincluding thermochromic microcapsules. The thermochromic microcapsulesare preferably present in an amount ranging from 1% to 50% of thecoating by weight on a sliding scale relative to other pigments. Thevehicle contains a solvent that is preferably present in an amountranging from 25% to 75% by weight of the coating.

The aqueous pigment slurries have particle sizes less than 5 microns andwhen drawn-down on coating test paper and dried, the pigment coatingshows reversible thermochromic properties when cooled to thesolidification point of the fatty ester, alcohol, amide, or a blenddesigned to obtain a specific temperature for full color formation. Suchpigments can be designed to have a range of temperature for transitionfrom full absorption temperature (full absorption color or UVAabsorption point) to no color or no UVA absorption temperature (clearingpoint) of 2-7° C. The pigments are very useful for manufacture ofcoating, and injected molded plastic products by spray drying prior toformulation into coating compositions or extrusion into thermoplasticpolymers to produce pellet concentrates for manufacture of injectionmolded thermochromic plastic products such as cups, cup lids, jars,straws, stirrers, container sleeves, shrink wrap labels. For example,thermochromic compositions were identified that permit generation ofhigh quality saturated photographic quality yellow color that is veryuseful to formulate new orange, red, and green colors by mixing withmagenta and/or cyan thermochromic pigments or by initialco-encapsulation of the yellow leuco dye with magenta and/or cyan leucodyes and appropriate color developers during the pigment manufacture.Alternatively leuco pigments were identified that can change fromabsorption mainly in the region from 280 to 350 nm to absorption mainlyfrom 350 to 400 nm.

Example 1 Pigment Formulations

The internal phase chemistry for the microcapsules has been tested withthe following solvents that to engineer the temperature profile andthermal memory:

Methyl Palmitate FC 12-13 CP 23-27 Tetradecanol FC 17-19 CP 29-33 LaurylLaurate FC 15-17 CP 25-29

These internal phase esters or alcohols have been tested with standardfluoran and phthalide dyes using BHPMP as a chemical developer. Theexact temperature profile and thermal memory is specific to the dye, ormixture of dyes. The dyes and developer may be co-encapsulated orseparately encapsulated to achieve a specific color with the desiredtemperature profile and thermal memory. The ratio of the dye:developermay be for example 1:1 to 1:4 in order to achieve desirable colordensity with minimal residual.

The following dyes may be microencapsulated with Developer CAS#6807-17-6(BHPMP) for various color formulations as described above.

Aqua dye CAS# 132467-74-4 Blue-63 dye CAS# 69898-40-4 Black XV CAS#36431-22-8 Red-40 dye CAS# 50292-91-6 Green dye CAS# 34372-72-0 Orangedye CAS# 21934-68-9

The internal phase as described above may be microencapsulated usingconventional urea-formaldehyde processes to form thermochromic pigments.

Example 2 Coating Formulations

Any of the thermochromic pigments prepared according to Example 1 abovemay be mixed with synthetic resins to form liquid coatings for use asprecursors in forming the layers 210-218. Various examples of thischemistry are as follows:

In one embodiment, a thermochromic coating formulation includes:

Ingredient Weight Percent of Coating Pigment* 1% to 40% VehiclePolymerizable resin 5% to 30% Dispersing agent 0% to 5%  Solvent 0% to50% Curing agent 0% to 25% Wax 0% to 5%  *Assessed by solids contentupon complete drying of pigment capsules, but does not need to be driedand may be mixed as a slurry.

In one aspect, a reversible thermochromic coating for use in can andcoil coatings contains a reversible thermochromic pigment in an amountfrom 1% to 50% by weight of the coating, and a vehicle forming thebalance of the coating. The vehicle includes a resin selected from thegroup consisting of epoxy, polyester, urethane, acrylic acid andacrylate resins, and combinations thereof. Commercially availablethermochromic pigments may be readily obtained in a variety of colorsdemonstrating color transition temperatures from about 5° C. and up toabout 65° C. A range of color formulations may be made by mixing thepigment to include one or more of the following reversible thermochromiccolors: yellow, magenta, cyan, and black. These may be further mixed toinclude other dyes or solid pigments that are non-thermochromic innature. The pigment may change from a colorless state to a colored stateupon cooling to the reactive temperature, or to a colored state uponheating to the reactive temperature. It is preferred that themicrocapsules are formed of urea-formaldehyde or melamine-formaldehydethat is acid catalyzed to enhance the inherent stability in polar, lowmolecular weight solvents having a molecular weight of about less than100 g/mol.

When premised using a nonpolar solvent, the coatings can demonstrateshelf stability exceeding 14 or 45 days when stored at about 20° C. Somecoating formulations demonstrate shelf stability in excess of one year.

The curing agent is generally compatible with the resin for this purposeand may be, for example, a latent blocked amine to initiate apolymerization reaction upon heating.

The coating is preferably roller-coated onto coil stock aluminum orsteel and the roll stock aluminum is subsequently formed into one ormore beverage can components. These components may be selected from thegroup consisting of beverage can ends, beverage can tabs, bottle caps,and/or beverage container closures. The aluminum is preferably an alloythat is commonly used in canning operations, such as aluminum alloy5182-H48. The coating process preferably occurs in one or more coats toyield a dried film with a thickness ranging from 1 mg/in² up to 5.5mg/in².

Example 3 Two Part Coating

Part A (30% by weight of coating)

Thermochromic pigment (any color)*

Part B (70% by weight of coating)

Clear Coating (an epoxy coating available from Watson Standard ofPittsburgh, Pa.)

* This material may be purchased on commercial order from ChromaticTechnologies, Inc. of Colorado Springs Colo., and may include forexample S5BOXX3105W, a blue thermochromic slurry that goes from acolored to colorless state when the temperature exceeds 31° C.

Example 4 Two Part Coating

Part A (60% by weight of coating)

45% Thermochromic Pigment (any color)*

50% Epoxy resin (for example Epon 863 available from Lawter of LaVergne,Tenn.)

3.3% Dispersing aid (for example Disperbyk 2025 available from Byk ofWallingford, Conn.

1.7% Curing agent (for example Ancamine 2458 available from Air Productsof Allentown, Pa.)

Part B (40% by weight of coating)

85% Clear Coating (an epoxy coating available from Watson Standard ofPittsburgh, Pa.)

15% Solvent to reduce viscosity (for example, butyl carbitol acetateavailable from Lawter of LaVergne, Tenn.)

* This material may be purchased on commercial order from ChromaticTechnologies, Inc. of Colorado Springs Colo., and may include forexample S5BOXX3105W, a blue thermochromic slurry that goes from acolored to colorless state when the temperature exceeds 31° C.

Example 5 One Part Coating

20% (w/w) Thermochromic Pigment (any color)* 13% Polyester resin (forexample, Decotherm 290 available from Lawter of LaVergne, Tenn.)

0.5% (w/w) Dispersing aid (for example, Byk 370 available from Byk ofWallingford, Conn.)

7% (w/w) Curing agent 1 (for example, Cymel 328 available from tecIndustries of Woodland Park, N.J.

1.5% (w/w) Curing agent 2 (for example, imidazole available from Aldrichof St. Louis, Mo.)

2% (w/w) Wax (for example, Fluoron 735 available from Lawter ofLaVergne, Tenn.)

30% (w/w) Solvent (for example, ethyl-3-ethoxypropionate available fromUnivar of Redmond, Wash.)

26% (w/w) Clear Coating (an epoxy coating available from Watson Standardof Pittsburgh, Pa.)

Example 6 One Part Coating

15% (w/w) Thermochromic Pigment (any color)*

10% (w/w) Resin (for example, Epon 896 available from Lawter ofLaVergne, Tenn.)

1.5% (w/w) Dispersing aid (for example, Disperbyk 112 available from Bykof Wallingford, Conn.

0.5% (w/w) Curing agent 1 (for example, Nacure 2500 available from KingIndustries of Norwalk, Conn.

4% (w/w) Curing agent 2 (for example, Cymel 325 available from CytecIndustries of Woodland Park, N.J.

1.5% (w/w) Wax—0.5 wt % (for example, Ultrapoly 211A available fromLawter of LaVergne, Tenn.)

5% (w/w) Solvent 1 (for example, Heloxy Modifier 62 available fromLawter of LaVergne, Tenn.)

21.5% (w/w) solvent 2 (for example, ethyl-3-ethoxypropionate availablefrom Univar of Redmond, Wash.)

41% (w/w) Clear Coating (an epoxy coating available from Watson Standardof Pittsburgh, Pa.)

Having described the invention in detail and by reference to preferredembodiments thereof, it will be apparent that modifications andvariations are possible without departing from the scope of theinvention defined in the appended claims.

We claim:
 1. A tape comprising; an elongate substrate having opposingfirst and second faces; an adhesive layer coupled to said first face; acolorant and a solvent which provide a first color, said colorant andsaid solvent encapsulated within a first pressure-rupturablemicrocapsule; and a developer; said first pressure-rupturablemicrocapsule and said developer combined with a first layer coupled tosaid first face, said first layer disposed between said first face andsaid adhesive layer; wherein, upon rupture of said firstpressure-rupturable microcapsule, said colorant and said solvent combinewith said developer to generate a color change from said first color toa second color.
 2. The tape of claim 1, wherein said developer isencapsulated within a second pressure-rupturable microcapsule, andwherein, upon rupture of said first and second pressure-rupturablemicrocapsules, said colorant and said solvent combine with saiddeveloper to generate said color change from said first color to saidsecond color.
 3. The tape of claim 2, wherein said color change isirreversible.
 4. The tape of claim 1, wherein said colorant comprises athermochromic compound, and wherein, upon rupture of said firstpressure-rupturable microcapsule, said thermochromic compound and saidsolvent combine with said developer to provide a thermochromic systemwhich exhibits said color change from said first color to said secondcolor upon a change in temperature.
 5. The tape of claim 4, wherein saidfirst color is colorless.
 6. The tape of claim 4, wherein said secondcolor is colorless.
 7. The tape of claim 4, wherein said developer isencapsulated within a second pressure-rupturable microcapsule, andwherein, upon rupture of said first and second pressure-rupturablemicrocapsules, said colorant and said solvent combine with saiddeveloper to generate said color change from said first color to saidsecond color.
 8. The tape of claim 7, wherein said color change isirreversible.
 9. The tape of claim 7, wherein said color change isreversible.
 10. The tape of claim 9, wherein said color change reversesupon a decrease in said temperature.
 11. The tape of claim 7, whereinsaid first and second pressure-rupturable microcapsules are combinedwith said at least one layer in a ratio having a range of between about1:1 to about 1:20 weight/weight.
 12. The tape of claim 7, wherein saidthermochromic compound comprises a leuco dye.
 13. The tape of claim 7,wherein said first and second pressure-rupturable microcapsules have amean diameter of less than about 5 micrometers.
 14. The tape of claim 7,wherein said first and second pressure-rupturable microcapsules have amean diameter of less than about 4 micrometers.
 15. The tape of claim 7,wherein said first and second pressure-rupturable microcapsules have amean diameter of less than about 3 micrometers.
 16. The tape of claim 7,wherein said elongate substrate is formed from a material selected fromthe group consisting of: paper, plastic, and combinations thereof.
 17. Amethod of masking a surface with a tape, said method comprising:obtaining said tape comprising: an elongate substrate having opposingfirst and second faces; an adhesive layer coupled to said first face; acolorant and a solvent which provide a first color, said colorant andsaid solvent encapsulated within a first pressure-rupturablemicrocapsule; and a developer; said first pressure-rupturablemicrocapsule and said developer combined with a first layer coupled tosaid first face, said first layer disposed between said first face andsaid adhesive layer; wherein, upon rupture of said firstpressure-rupturable microcapsule, said colorant and said solvent combinewith said developer to generate a color change from said first color toa second color; overlaying said adhesive layer coupled to said firstface of said elongate substrate on said surface; applying forces to saidsecond face of said elongate substrate to adhere said tape to saidsurface wherein said forces rupture said first pressure-rupturablemicrocapsule; and observing said color change from said first color tosaid second color, said color change indicating that said tape issufficiently adhered to said surface to mask said surface.
 18. A methodof masking a surface with a tape, said method comprising: obtaining saidtape comprising: an elongate substrate having opposing first and secondfaces; an adhesive layer coupled to said first face; a colorant and asolvent which provide a first color, said colorant and said solventencapsulated within a first pressure-rupturable microcapsule; and adeveloper encapsulated within a second pressure-rupturable microcapsule;said first and second pressure-rupturable microcapsules combined with afirst layer coupled to said first face, said first layer disposedbetween said first face and said adhesive layer; wherein, upon ruptureof said first and second pressure-rupturable microcapsules, saidcolorant and said solvent combine with said developer to generate acolor change from said first color to a second color; overlaying saidadhesive layer coupled to said first face of said elongate substrate onsaid surface; applying forces to said second face of said elongatesubstrate to adhere said tape to said surface wherein said forcesrupture said first and second pressure-rupturable microcapsules; andobserving said color change from said first color to said second color,said color change indicating that said tape is sufficiently adhered tosaid surface to mask said surface.